Within the context of the production of crude petroleum, or oil, from subterranean formations, there are already various methods in existence for optimizing the extraction of original oil in place (OOIP).
The primary method of crude oil production consists, once the well has been drilled, of recovering the crude oil by migration of the oil from the rock or sand formation to a lower pressure “producer” well. Consequently the primary production is the least expensive method of extraction. Typically only 10 to 15% of OOIP are recovered. Nevertheless, as the petroleum is pumped, the pressure of the reservoir decreases and the extraction becomes more difficult.
Secondary methods of production are employed when the subterranean pressure becomes insufficient to displace the remaining petroleum. The most common technique, water-flooding, uses injection wells which force a driving fluid consisting of large volumes of water under pressure into the zone containing the petroleum. During its migration from the zone towards one or more producer wells, the injected water extracts part of the petroleum which it encounters. On the surface, the petroleum is separated from the injected water. Water-flooding makes it possible to recover an additional 10 to 30% of OOIP.
When the water-flooding reaches the point where the production is no longer profitable, a decision should be taken: to change the oil field, or to use another phase of exploitation. It is then possible to use a technique of assisted recovery using water-flooding in which the water comprises surfactants and/or polymers. These polymers are used in order to increase the viscosity of the driving fluid and thus to improve the scavenging of the petroleum by the driving fluid. It is known for example to increase the viscosity of the water by means of viscosifiers such as partially hydrolyzed polyacrylamides of high molecular weight. These polymers, dispersible and/or soluble in water, enable more homogeneous scavenging of the formation while reducing the appearance of viscous fingering.
Zwitterionic surfactants and in particular betaines can be used as viscosifiers because of their stability in brines of which the temperature can reach 80° C. and more. The term zwitterionic describes surfactants having a permanent positive charge independently of the pH and having a negative charge above a certain pH.
In another approach, upon contact with the petroleum contained in the rock or the sand the surfactants lower the water/oil interfacial tension in order to make it possible to entrain the oil trapped in the constrictions of the pores of the reservoir.
However, numerous hydrocarbon reservoirs are not homogeneous with regard to geology and exhibit a great variety of porosity and of permeability. This is the case in particular for carbonate reservoirs which are often naturally and abundantly fractured. These fractured reservoirs include two separate elements: fractures and a matrix consisting of the rock containing petroleum. If the network of fractures permits an easy transfer of fluids from the reservoir, it only represents a small fraction of the total porosity of the reservoir. The matrix which has a low permeability contains the majority of the porosity of the reservoir and therefore of the oil. The water invasion techniques mentioned above (“water-flooding”) and applied to these reservoir rapidly lead to an excessive increase in the water/oil ratio at the producer well. After having entrained the petroleum located in the fractures, the water continues to be displaced preferentially through the network of fractures and, in so doing, does not entrain the petroleum of the matrix. This results from the fact that the matrix is most often preferentially wettable with the oil as is often the case with carbonate reservoirs. Therefore water cannot spontaneously soak the matrix in order to displace the oil which is trapped there and causes the fractures, the pressure loss between the injector well and the producer well, both in communication with the fractures and the matrix, is too low to force this soaking.
A known method known for increasing the penetration of water into the matrix consists of injecting an aqueous solution of specific surfactant with the purpose of creating favorable conditions for wetting with water. In order to take advantage of this method, it is recommended to proceed with cyclic injection of fluids, a method described under the term “cyclic wettability alteration” as illustrated in the patents U.S. Pat. Nos. 4,842,065, 5,014,783 and 5,247,993. The principle consists in the first place of injecting a solution of surfactants capable of altering the wettability of the matrix, in order next to observe a latency before injecting water. By repeating these steps in this way part of the oil in the matrix is displaced. However, the water/oil ratios remain insufficient and these processes necessitate repeated injections in quick succession of surfactant and water.
Therefore there is a need to improve these processes in order to achieve more favorable oil/water ratios.